Generally, in LNG carriers, which store LNG at a cryogenic temperature (approximately −163° C.) in LNG cargo tanks and carry the LNG, the cargo tanks must have special structures, in consideration of the problem of the brittle fracture of a carrier bodies due to the cryogenic temperature of the LNG. The term ‘special structure’ means a structure for insulating and isolating cryogenic LNG from structural members of the carrier body or cargo tank. Typically, as the special structures, membrane structures, in which barriers having superior low-temperature resistance are provided in the structural members of the carrier body and insulating substances are interposed between the barriers, have been widely used.
FIG. 1 is a view showing a typical LNG carrier having cargo tanks. As shown in FIG. 1, in the LNG carrier 1, a plurality of cargo tanks 3 occupies almost all of the space in the carrier body, other than crew quarters 5, in which sailors reside, and a power generating part 7, which generates driving force for propelling the LNG carrier 1.
The cargo tanks 3 define therein space for storing LNG to be carried by the LNG carrier 1. A cofferdam 9, that is, a space between adjacent cargo tanks 3, is provided. The cofferdam 9 has a device for heating air therein, and thus serves to prevent the wall of the carrier body from being damaged by exchanging heat with the cargo tanks 3, which contain LNG of a cryogenic temperature therein.
FIG. 2 is a cross sectional view showing a conventional membrane type cargo tank. As shown in FIG. 2, the internal structure of the membrane type cargo tank 3 includes a carrier body inner wall 10, which is made of carbon steel, and an insulating compound barrier layer 20, which is provided on the inner surface of the carrier body inner wall 10.
The insulating compound barrier layer 20 conducts an insulating and isolating function for preventing the carrier body inner wall 10 from being damaged by LNG of a cryogenic temperature. A first barrier 22, made of stainless steel (SUS), is provided in the insulating compound barrier layer 20 at the innermost position, which is closest to the internal space of the cargo tank 3, that is, a position at which it comes into direct contact with LNG. A first insulation pad 24 is provided on the outer surface of the first barrier 22. A second barrier 26, which is made of triplex material and supplements the function of the first barrier 22, is provided on the outer surface of the first insulation pad 24. A second insulation pad 28 is provided on the outer surface of the second barrier 26. The second insulation pad 28 is in close contact with the carrier body inner wall 10, which is one of the structural members of the LNG carrier. This structure prevents the carrier body inner wall 10 from being damaged by the cryogenic LNG that is contained in the cargo tank 3.
However, in the case of marine structures, such as an LNG carrier (LNGC), which travels on the sea, and a floating storage regasification unit (FSRU), which is used at a stationary location, the structure shakes depending on sea conditions, for example, due to waves or sea wind. When the structure shakes, LNG that contained in the cargo tank 3 also moves therein. At this time, the moving LNG strikes the inner wall of the cargo tank 3. This phenomenon is called ‘sloshing’, and an impact applied to the inner wall by the sloshing is called a sloshing impact.
The damage to the inner wall of the cargo tank 3, attributable to the sloshing, increases as the size of the cargo tank 3 is increased. This limits the size of the cargo tank 3. Particularly, in the case of an FSRU, which is used in a state in which it is anchored for a long period of time at a specific location at sea, or an LNG carrier traveling in poor conditions, the above-mentioned limitation attributable to the sloshing becomes an important design factor.
Therefore, there is required an anti-sloshing LNG cargo tank applicable to a large LNG cargo tank of an LNG carrier or an FSRU which stores and manages LNG.